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To W. E. Darwin [1 March 1872]1

9 Dev: St—

Friday

My dear Wm

It was a capital thought of yours, to get the chalk analysed.2 Immediately on getting your note I called on David Forbes, who is the one man in England who has carefully analysed many specimens of chalk.3 He says that from 1 to 2 per cent of earthy matter is 〈a〉 fair average for the upper chalk with flints. In the lowest & most earthy beds of chalk without flints there is as much as 4 or 5 per cent. Therefore he concludes that the specimens analysed by Capt Parsons must certainly have become penetrated with surface mud.4 This seems to remove the wonderful difficulty of the layer of mould not being thicker at the bottom of your valley. I suppose the chalk about Winchester must be very pure, or unusually porous; for about Down the residue from the dissolved chalk evidently accumulates on the surface, & does not to any great extent percolate into the solid mass—